Semiconductor device

ABSTRACT

According to one embodiment, a semiconductor device includes a first crystal region, a second crystal region, a third crystal region, and a fourth crystal region. The first crystal region includes magnesium and Alx1Ga1-x1N (0≤x1&lt;1). The second crystal region includes Alx2Ga1-x2N (0&lt;x2≤1). The third crystal region is provided between the first crystal region and the second crystal region. The third crystal region includes oxygen and Alx3Ga1-x3N (0≤x3≤1 and x3&lt;x2). The fourth crystal region is provided between the third crystal region and the second crystal region. The fourth crystal region includes Alx4Ga1-x4N (0≤x4&lt;1 and x4&lt;x2).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims benefit under 35 U.S.C. §120 to U.S. application Ser. No. 17/141,269 filed on Jan. 5, 2021, whichis based upon and claims the benefit of priority under 35 U.S.C. § 119to Japanese Patent Application No. 2020-068619, filed on Apr. 6, 2020,the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein generally relate to a semiconductor device.

BACKGROUND

For example, it is desirable to improve the characteristics of asemiconductor device such as a transistor or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a semiconductordevice according to a first embodiment;

FIG. 2 is a graph illustrating the semiconductor device according to thefirst embodiment;

FIGS. 3A and 3B are graphs illustrating characteristics of thesemiconductor device;

FIG. 4 is a graph illustrating characteristics of the semiconductordevice;

FIG. 5 is a graph illustrating characteristics of semiconductor devices;

FIG. 6 is a schematic cross-sectional view illustrating a semiconductordevice according to the first embodiment;

FIG. 7 is a schematic cross-sectional view illustrating a semiconductordevice according to a second embodiment;

FIG. 8 is a schematic cross-sectional view illustrating a semiconductordevice according to the second embodiment; and

FIG. 9 is a schematic cross-sectional view illustrating a semiconductordevice according to the second embodiment.

DETAILED DESCRIPTION

According to one embodiment, a semiconductor device includes a first crregion, a second crystal region, a third crystal region, and a fourthcrystal region. The first crystal region includes magnesium andAl_(x1)Ga_(1-x1)N (0≤x1<1). The second crystal region includesAl_(x2)Ga_(1-x2)N (0<x2≤1). The third crystal region is provided betweenthe first crystal region and the second crystal region. The thirdcrystal region includes oxygen and Al_(x3)Ga_(1-x3)N (0≤x3≤1 and x3<x2).The fourth crystal region is provided between the third crystal regionand the second crystal region. The fourth crystal region includesAl_(x4)Ga_(1-x4)N (0≤x4<1 and x4<x2).

According to one embodiment, a semiconductor device includes a firstcrystal region, a second crystal region, a third crystal region, and afourth crystal region. The first crystal region includes magnesium andAl_(x1)Ga_(1-x1)N (0≤x1<1). The second crystal region includesAl_(x2)Ga_(1-x2)N (0<x2≤1 and x1<x2). The third crystal region isprovided between the first crystal region and the second crystal region.The third crystal region includes silicon and Al_(x3)Ga_(1-x3)N(0≤x3≤1). The fourth crystal region is provided between the thirdcrystal region and the second crystal region. The fourth crystal regionincludes Al_(x4)Ga_(1-x4)N (0≤x4<1 and x4<x2). The third crystal regionincludes silicon. The first crystal region does not include silicon, ora concentration of silicon in the first crystal region is less than aconcentration of silicon in the third crystal region.

Various embodiments are described below with reference to theaccompanying drawings.

The drawings are schematic and conceptual; and the relationships betweenthe thickness and width of portions, the proportions of sizes amongportions, etc., are not necessarily the same as the actual values. Thedimensions and proportions may be illustrated differently amongdrawings, even for identical portions.

In the specification and drawings, components similar to those describedpreviously in an antecedent drawing are marked with like referencenumerals, and a detailed description is omitted as appropriate.

First Embodiment

FIG. 1 is a schematic cross-sectional view illustrating a semiconductordevice according to a first embodiment.

As shown in FIG. 1 , the semiconductor device 110 according to theembodiment includes a first crystal region 11, a second crystal region12, a third crystal region 13, and a fourth crystal region 14.

The first crystal region 11 includes magnesium (Mg) andAl_(x1)Ga_(1-x1)N (0≤x1<1). The composition ratio x1 is, for example,not less than 0 and not more than 0.5. In one example, the compositionratio x1 is, for example, not less than 0 but less than 0.2. The firstcrystal region 11 is, for example, a GaN layer.

The second crystal region 12 includes Al_(x2)Ga_(1-x2)N (0<x2≤1). Thecomposition ratio x2 is, for example, not less than 0.1 and not morethan 0.5. In one example, the composition ratio x2 is, for example, notless than 0.2 and not more than 0.5. For example, the composition ratiox2 is greater than the composition ratio x1. In another example, thecomposition ratio x2 is, for example, not less than 0.1 and not morethan 0.2. The second crystal region 12 is, for example, an AlGaN layer.

The third crystal region 13 is located between the first crystal region11 and the second crystal region 12. The third crystal region 13includes oxygen and Al_(x3)Ga_(1-x3)N (0≤x3≤1 and x3<x2). Thecomposition ratio x3 is, for example, not less than 0 and not more than0.5. In one example, the composition ratio x3 is, for example, not lessthan 0 but less than 0.2. In another example, the composition ratio x3is, for example, not less than 0 but less than 0.1. The third crystalregion 13 is, for example, a GaN layer that includes oxygen. The thirdcrystal region 13 maintains the crystal structure of the nitridesemiconductor while including oxygen. In one example of the thirdcrystal region 13, the oxygen may be at lattice points of the crystalstructure. In another example of the third crystal region 13, the oxygenmay be at a position different from lattice points of the crystalstructure. The concentration (e.g., the peak concentration) of oxygen inthe third crystal region 13 is, for example, not less than 2×10¹⁷ cm⁻³and not more than 1×10²⁰ cm⁻³.

The fourth crystal region 14 is located between the third crystal region13 and the second crystal region 12. The fourth crystal region 14includes Al_(x4)Ga_(1-x4)N (0≤x4<1 and x4<x2). The fourth crystal region14 is, for example, a GaN layer. In one example, the composition ratiox4 is, for example, not less than 0 but less than 0.2. In anotherexample, the composition ratio x4 is, for example, not less than 0 butless than 0.1.

As shown in FIG. 1 , the semiconductor device 110 may include asubstrate 10 s and a sixth crystal region 16. For example, the sixthcrystal region 16 includes a nitride semiconductor that includes Al. Thesixth crystal region 16 is, for example, a buffer layer. The sixthcrystal region 16 is between the substrate 10 s and the first crystalregion 11. For example, the sixth crystal region 16 is provided on thesubstrate 10 s. The first crystal region 11 is provided on the sixthcrystal region 16. The third crystal region 13 is provided on the firstcrystal region 11. The fourth crystal region 14 is provided on the thirdcrystal region 13. The second crystal region 12 is provided on thefourth crystal region 14.

The direction from the first crystal region 11 toward the fourth crystalregion 14 is taken as a Z-axis direction. One direction perpendicular tothe Z-axis direction is taken as an X-axis direction. A directionperpendicular to the Z-axis direction and the X-axis direction is takenas a Y-axis direction. The Z-axis direction corresponds to the stackingdirection of the first to fourth crystal regions 11 to 14. The first tofourth crystal regions 11 to 14 spread along the X-Y plane.

In one example as described below, the semiconductor device 110 is, forexample, at least a portion of a transistor. For example, a carrierlayer (e.g., a two-dimensional electron gas) is formed at the vicinityof the interface between the second crystal region 12 of the fourthcrystal region 14. In the semiconductor device 110, the thresholdvoltage can be increased by providing the first crystal region 11including Mg. It was found that by providing the third crystal region13, for example, the Mg concentration in the fourth crystal region 14can be reduced. By reducing the Mg concentration in the fourth crystalregion 14, for example, a high mobility can be maintained. For example,a low on-resistance is obtained. For example, a high threshold and a lowon-resistance are obtained. According to the embodiment, a semiconductordevice can be provided in which the characteristics can be improved.

For example, the sixth crystal region 16, the first crystal region 11,and the third crystal region 13 are formed by crystal growth in thisorder on the substrate 10 s. The fourth crystal region 14 and the secondcrystal region 12 are formed by crystal growth in this order on thethird crystal region 13. It is considered that the third crystal region13 suppresses the movement of the Mg included in the first crystalregion 11 toward the fourth crystal region 14.

In the embodiment, for example, the fourth crystal region 14 does notinclude Mg. Or, a fourth concentration of Mg in the fourth crystalregion 14 is less than a first concentration of Mg in the first crystalregion 11. For example, the fourth concentration is not more than ⅕ ofthe first concentration.

An example of profiles of the concentrations of Mg, etc., of thesemiconductor device 110 will now be described.

FIG. 2 is a graph illustrating the semiconductor device according to thefirst embodiment.

The horizontal axis of FIG. 2 is a position pZ along the Z-axisdirection. The vertical axis of FIG. 2 is a logarithm CMg of the Mgconcentration. FIG. 2 shows a logarithm CMg1 of the first concentrationof Mg in the first crystal region 11, a logarithm CMg2 of a secondconcentration of Mg in the second crystal region 12, a logarithm CMg3 ofa third concentration of Mg in the third crystal region 13, and alogarithm CMg4 of the fourth concentration of Mg in the fourth crystalregion 14.

As shown in FIG. 2 , the fourth concentration (the logarithm CMg4) of Mgin the fourth crystal region 14 is less than the first concentration(the logarithm CMg1) of Mg in the first crystal region 11. The thirdconcentration (the logarithm CMg3) of Mg in the third crystal region 13decreases along a first orientation (the +Z orientation) from the firstcrystal region 11 toward the fourth crystal region 14. A third changerate of the logarithm CMg3 of the third concentration with respect tothe change of the position pZ along the first orientation is greaterthan a fourth change rate of the logarithm CMg4 of the fourthconcentration with respect to the change of the position pZ along thefirst orientation. The concentration of Mg abruptly decreases in thethird crystal region 13.

The embodiment includes the third crystal region 13 in which the Mgconcentration abruptly changes, and the fourth crystal region 14 inwhich the change rate of the Mg concentration is small (or does notchange). For example, a two-dimensional electron gas is formed in thefourth crystal region 14 at the vicinity of the second crystal region12. When the Mg concentration in the fourth crystal region 14 is high,the movement of the electrons is impeded by the Mg. By setting the Mgconcentration in the fourth crystal region 14 to be low, the impedanceof the movement of the electrons by the Mg is suppressed. By setting theMg concentration in the fourth crystal region 14 to be low, highelectron mobility is easily obtained.

By setting the Mg concentration in the first crystal region 11 to behigh, the threshold voltage can be effectively increased. Theconcentration of Mg abruptly decreases in the third crystal region 13.

It is considered that the movement of Mg in the third crystal region 13is suppressed by, for example, a strong bond between Mg and oxygen. Forexample, it is considered that in the growth of the third crystal region13, the Mg diffuses upward due to the oxygen included in the firstcrystal region 11 being replaced with Ga of the surface of the thirdcrystal region 13. Because the third crystal region 13 includes oxygen,the oxygen and the Mg bond at this time. It is considered that theupward diffusion of the Mg is suppressed thereby.

As shown in FIG. 2 , the second concentration (the logarithm CMg2) of Mgin the second crystal region 12 is less than the first concentration(the logarithm CMg1). For example, the second concentration (thelogarithm CMg2) is less than the fourth concentration (the logarithmCMg4).

FIGS. 3A, 3B, and 4 are graphs illustrating characteristics of thesemiconductor device.

These figures illustrate SIMS (Secondary Ion Mass Spectrometry) analysisresults of a sample according to the embodiment. In this sample, thesixth crystal region 16 and a first layer, which is used to form thefirst crystal region 11, are formed by crystal growth in this order onthe substrate 10 s. Subsequently, a second layer that includes oxygen isformed on the first layer that is used to form the first crystal region11, and the second layer is subsequently removed. In the example, thesecond layer includes Si in addition to oxygen. At least a portion ofthe oxygen and the silicon included in the second layer is introduced tothe first layer. The region of the first layer into which the oxygen(and the silicon) are introduced is used to form the third crystalregion 13. The region of the first layer into which oxygen (and silicon)are substantially not introduced is used to form the first crystalregion 11. The third crystal region 13 that stably includes oxygen (andsilicon) can be stably formed by such a method. The sample is made byforming the fourth crystal region 14 and the second crystal region 12 onthe third crystal region 13.

FIG. 3A shows a concentration CMg0 of Mg and a concentration CO0 ofoxygen of the sample. FIG. 3B shows the concentration CMg0 of Mg and aconcentration CSi0 of silicon (Si). The horizontal axis is the positionpZ in the Z-axis direction. The vertical axis is a concentration C1.

As shown in FIGS. 3A and 3B, the concentration CMg0 of Mg decreases in astep configuration in the third crystal region 13. The concentrationCMg0 of Mg is low in the fourth crystal region 14. As shown in FIG. 3A,the concentration CO0 of oxygen is locally high in the third crystalregion 13. By providing the third crystal region 13 that has thelocally-high concentration of Mg, the concentration CMg0 of Mg in thethird crystal region 13 is caused to decrease in the step configuration.

As shown in FIG. 3B, the concentration CSi0 of Si may be locally high inthe third crystal region 13. For example, there are cases where oxygenfunctions as a donor in a nitride semiconductor. By providing the thirdcrystal region 13 that has the locally-high concentration of Si, theoxygen that is included in the third crystal region 13 is deactivated.For example, bond between silicon and oxygen exist in the third crystalregion 13. The unfavorable effects of oxygen in the third crystal region13 can be suppressed thereby.

Thus, the third crystal region 13 may include silicon. The first crystalregion 11 does not include silicon. Or, the silicon concentration in thefirst crystal region 11 is less than the silicon concentration in thethird crystal region 13. For example, the fourth crystal region 14 doesnot include silicon. Or, the silicon concentration in the fourth crystalregion 14 is less than the silicon concentration in the third crystalregion 13.

For example, the concentration (e.g., the peak concentration) of siliconin the third crystal region 13 is not less than 2×10¹⁷ cm⁻³ and not morethan 5×10¹⁹ cm⁻³.

FIG. 4 shows the concentration CMg0 of Mg and a concentration CC0 ofcarbon of the sample. The horizontal axis is the position pZ in theZ-axis direction. The vertical axis is the concentration C1. As shown inFIG. 4 , the concentration CC0 of carbon is locally high in the thirdcrystal region 13. It is considered that the carbon originates in anorganic metal, which is a source gas, etc. For example, carbon has afunction of compensating residual donors. By compensating the residualdonors, unfavorable effects of oxygen and/or silicon can be suppressed.

Thus, the third crystal region 13 may include carbon. The first crystalregion 11 does not include carbon. Or, the carbon concentration in thefirst crystal region 11 is less than the carbon concentration in thethird crystal region 13. For example, the fourth crystal region 14 doesnot include carbon. Or, the carbon concentration in the fourth crystalregion 14 is less than the carbon concentration in the third crystalregion 13.

For example, the concentration (e.g., the peak concentration) of carbonin the third crystal region 13 is not less than 1×10¹⁷ cm⁻³ and not morethan 1×10²⁰ cm⁻³.

In the embodiment, for example, at least a portion of the crystal of thethird crystal region 13 is continuous with the crystal of the firstcrystal region 11. For example, the third crystal region 13 is anepitaxial layer that is provided on the fourth crystal region 14. Forexample, at least a portion of the crystal of the third crystal region13 is continuous with the crystal of the fourth crystal region 14. Forexample, the fourth crystal region 14 is an epitaxial layer that isprovided on the third crystal region 13. By providing a crystal regionin which the crystal is continuous, for example, high crystal quality isobtained. For example, high mobility is obtained.

An example of characteristics of the semiconductor device 110 (e.g., thetransistor) will now be described.

FIG. 5 is a graph illustrating characteristics of semiconductor devices.

FIG. 5 shows measurement results of characteristics of a semiconductordevice 119 of a reference example in addition to measurement results ofthe characteristics of the semiconductor device 110 according to theembodiment. The third crystal region 13 is not provided in thesemiconductor device 119. The first crystal region 11 and the fourthcrystal region 14 contact each other in the semiconductor device 119.Otherwise, the configuration of the semiconductor device 119 is similarto the configuration of the semiconductor device 110. In thesemiconductor device 119, the fourth crystal region 14 and the secondcrystal region 12 are continuously formed after forming the first layerthat is used to form the first crystal region 11. In the semiconductordevice 119, the Mg concentration in the second crystal region 12 is notless than about 2×10¹⁸ cm⁻³ and not more than about 6×10¹⁸ cm⁻³, and theMg concentration in the fourth crystal region 14 is not less than about1×10¹⁸ cm⁻³ and not more than about 5×10¹⁸ cm⁻³. The semiconductordevices 110 and 119 are transistors.

The horizontal axis of FIG. 5 is a carrier density CD. The vertical axisis a carrier mobility μ. When comparing the same carrier density CD asshown in FIG. 5 , the mobility of the semiconductor device 110 isgreater than that of the semiconductor device 119. It is considered thatthis is caused by the low Mg concentration in the second and fourthcrystal regions 12 and 14 of the semiconductor device 110. According tothe embodiment, a high carrier mobility μ is obtained. A lowon-resistance is obtained thereby.

Thus, according to the embodiment, a high threshold is obtained byproviding the first crystal region 11 that includes Mg. By providing thethird crystal region 13, the Mg concentration in the second and fourthcrystal regions 12 and 14 can be low. For example, a low on-resistanceis obtained.

In the embodiment, it is favorable for a thickness t3 of the thirdcrystal region 13 (referring to FIG. 1 ) to be, for example, not lessthan 2 nm and not more than 20 nm. The third crystal region 13 is, forexample, a delta-doped layer of oxygen (and silicon). The thickness t3may be not less than 2 nm and not more than 10 nm. By setting thethickness t3 to be not less than 2 nm, for example, the diffusion (orthe movement) of Mg is easily suppressed. By setting the thickness t3 tobe not more than 20 nm, for example, a high threshold voltage is easilyobtained. By setting the thickness t3 to be not more than 10 nm, forexample, an even higher threshold voltage is easily obtained.

It is favorable for a thickness t4 of the fourth crystal region 14(referring to FIG. 1 ) to be, for example, not less than 10 nm and notmore than 200 nm. By setting the thickness t4 to be not less than 10 nm,for example, the threshold voltage is stably and easily controlled. Bysetting the thickness t4 to be not more than 100 nm, for example, a highthreshold voltage is easily obtained.

It is favorable for a thickness t1 of the first crystal region 11(referring to FIG. 1 ) to be, for example, not less than 200 nm and notmore than 1000 nm. By setting the thickness t1 to be not less than 200nm, for example, a high threshold voltage is easily obtained. By settingthe thickness t1 to be not more than 1000 nm, for example, the residualMg at the interior wall of the crystal growth processing chamber or thelike is easily suppressed. The incorporation of Mg from the vapor phasecan be suppressed thereby.

It is favorable for a thickness t2 of the second crystal region 12(referring to FIG. 1 ) to be, for example, not less than 1 nm and notmore than 50 nm. By setting the thickness t2 to be not less than 1 nm,for example, high electron mobility is stably and easily obtained, Bysetting the thickness t1 to be not more than 50 nm, for example, goodMIS structure gate characteristics are easily obtained. By setting thethickness t2 to be not more than 10 nm, for example, a high thresholdvoltage is easily obtained. The thickness t1, the thickness t2, thethickness t3, and the thickness t4 are, for example, lengths along theZ-axis direction.

FIG. 6 is a schematic cross-sectional view illustrating a semiconductordevice according to the first embodiment.

As shown in FIG. 6 , the semiconductor device 111 according to theembodiment includes a fifth crystal region 15 in addition to the first,second, third, and fourth crystal regions 11, 12, 13, and 14. Otherwise,the configuration of the semiconductor device 111 may be similar to theconfiguration of the semiconductor device 110. An example of the fifthcrystal region 15 will now be described.

As shown in FIG. 6 , the fifth crystal region 15 is provided between thefirst crystal region 11 and the third crystal region 13. The fifthcrystal region 15 includes Al_(x5)Ga_(1-x5)N (0<x5≤1, x1<x5, and x3<x5).The composition ratio x5 is, for example, not less than 0.1 and not morethan 0.5. In one example, the composition ratio x5 is, for example, notless than 0.2 and not more than 0.5. The fifth crystal region 15 is, forexample, an AlGaN layer.

The surface of the AlGaN layer easily oxidizes. The surface of the AlGaNlayer easily incorporates oxygen. For example, the region of the AlGaNlayer that incorporates oxygen may correspond to at least a portion ofthe third crystal region 13. For example, the region of the AlGaN layerinto which oxygen is substantially not incorporated may correspond tothe fifth crystal region 15. In the third crystal region 13, thecomposition ratio of Al may change along the Z-axis direction. Byproviding the fifth crystal region 15, a region that locally includesoxygen is stably and easily obtained.

For example, at least a portion of the crystal of the third crystalregion 13 is continuous with the crystal of the fifth crystal region 15.For example, at least a portion of the crystal of the fifth crystalregion 15 is continuous with the crystal of the first crystal region 11.

Second Embodiment

FIG. 7 is a schematic cross-sectional view illustrating a semiconductordevice according to a second embodiment.

As shown in FIG. 7 , the semiconductor device 120 according to theembodiment includes a first electrode 51, a second electrode 52, and athird electrode 53 in addition to the substrate 10 s, the first crystalregion 11, the third crystal region 13, the fourth crystal region 14,and the second crystal region 12. In the example, the semiconductordevice 120 includes the sixth crystal region 16. The configurations ofthe first, third, fourth, and second crystal regions 11, 13, 14, and 12of the semiconductor device 120 may be the same respectively as theconfigurations of the semiconductor device 110. The fifth crystal region15 may be provided in the semiconductor device 120. An example of theelectrodes will now be described.

As shown in FIG. 7 , the direction from a portion 10 a of the firstcrystal region 11 toward the first electrode 51 is along the firstdirection (e.g., the Z-axis direction) from the first crystal region 11toward the second crystal region 12. The direction from another portion10 b of the first crystal region 11 toward the second electrode 52 isalong the first direction (the Z-axis direction). A second directionfrom the first electrode 51 toward the second electrode 52 crosses thefirst direction. The second direction is, for example, the X-axisdirection. The position in the second direction of the third electrode53 is between the position in the second direction of the firstelectrode 51 and the position in the second direction of the secondelectrode 52.

For example, in the example, a portion of the second crystal region 12is between the third electrode 53 and a portion 10 c of the firstcrystal region 11. A portion of the second crystal region 12 is betweenthe third electrode 53 and a portion of the fourth crystal region 14. Aninsulating film 80 is between the second crystal region 12 and the thirdelectrode 53.

For example, the first electrode 51 functions as a source electrode. Forexample, the second electrode 52 functions as a drain electrode. Forexample, the third electrode 53 functions as a gate electrode. Thesemiconductor device 120 is, for example, a HEMT (High Electron MobilityTransistor). In the semiconductor device 120, for example, normally-offcharacteristics are easily obtained when the composition ratio x2 islow, i.e., not less than 0.1 and not more than 0.2.

FIGS. 8 and 9 are schematic cross-sectional views illustratingsemiconductor devices according to the second embodiment.

As shown in FIGS. 8 and 9 , the semiconductor devices 121 and 122according to the embodiment also include the substrate 10 s, the firstcrystal region 11, the third crystal region 13, the fourth crystalregion 14, the second crystal region 12, the first electrode 51, thesecond electrode 52, the third electrode 53, and the insulating film 80.

In the semiconductor devices 121 and 122, the direction from at least aportion of the third electrode 53 toward the second crystal region 12 isalong the X-axis direction. The semiconductor devices 121 and 122include, for example, recessed gate electrodes. In the semiconductordevice 121, the direction from at least a portion of the third electrode53 toward a portion of the second crystal region 12 is along the X-axisdirection. In the semiconductor device 122, the direction from at leasta portion of the third electrode 53 toward a portion of the fourthcrystal region 14 is along the X-axis direction.

The first crystal region 11 described above is grown using a gas thatincludes a raw material including gallium, ammonia, and a raw materialincluding magnesium.

As described above, the third crystal region 13 can be formed byexposing the first layer that is used to form the first crystal region11 to a material (an atmosphere) including oxygen. Or, at least aportion of the formation of the third crystal region 13 may be performedusing a gas that includes oxygen, ammonia, and a raw material includinggallium.

The fourth crystal region 14 described above is grown using a gasincluding a raw material that includes ammonia and a raw materialincluding gallium. The second crystal region 12 described above is grownusing a gas that includes a raw material including gallium and aluminumand a raw material including ammonia.

In the embodiment, the substrate 10 s includes, for example, silicon.The substrate 10 s may include, for example, sapphire, SiC, or GaN. Thesixth crystal region 16 includes, for example, AlN. The sixth crystalregion 16 may include, for example, a stacked body in which multipleAlGaN layers are stacked. The sixth crystal region 16 may include, forexample, a superlattice structure in which a GaN layer and an AlN layerare periodically stacked.

According to the embodiments, a semiconductor device can be provided inwhich the characteristics can be improved.

In the embodiments, “nitride semiconductor” includes all compositions ofsemiconductors of the chemical formula B_(x)In_(y)Al_(z)Ga_(1-x-y-z)N(0≤x≤1, 0≤y≤1, 0≤z≤1, and x+y+z≤1) for which the composition ratios x,y, and z are changed within the ranges respectively, “Nitridesemiconductor” further includes Group V elements other than N (nitrogen)in the chemical formula recited above, various elements added to controlvarious properties such as the conductivity type and the like, andvarious elements included unintentionally.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the embodiments of theinvention are not limited to these specific examples. For example, oneskilled in the art may similarly practice the invention by appropriatelyselecting specific configurations of components included insemiconductor devices such as substrates, crystal regions, electrodes,insulating films, etc., from known art. Such practice is included in thescope of the invention to the extent that similar effects thereto areobtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all semiconductor devices practicable by an appropriate designmodification by one skilled in the art based on the semiconductordevices described above as embodiments of the invention also are withinthe scope of the invention to the extent that the spirit of theinvention is included.

Various other variations and modifications can be conceived by thoseskilled in the art within the spirit of the invention, and it isunderstood that such variations and modifications are also encompassedwithin the scope of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. A semiconductor device, comprising: a firstcrystal region including magnesium and Al_(x1)Ga_(1-x1)N (0≤x1<1); asecond crystal region including Al_(x2)Ga_(1-x2)N (0<x2≤1 and x1<x2); athird crystal region provided between the first crystal region and thesecond crystal region, the third crystal region including silicon andAl_(x3)Ga_(1-x3)N (0≤x3≤1); and a fourth crystal region provided betweenthe third crystal region and the second crystal region, the fourthcrystal region including Al_(x4)Ga_(1-x4)N (0≤x4<1 and x4<x2), the thirdcrystal region including silicon, the first crystal region not includingsilicon, or a concentration of silicon in the first crystal region beingless than a concentration of silicon in the third crystal region.
 2. Thedevice according to claim 1, wherein the concentration of silicon in thethird crystal region is not less than 2×10¹⁷ cm⁻³ and not more than5×10¹⁹ cm⁻³.
 3. The device according to claim 1, wherein a concentrationof oxygen in the third crystal region is not less than 2×10¹⁷ cm⁻³ andnot more than 1×10²⁰ cm⁻³.
 4. The device according to claim 1, whereinthe fourth crystal region does not include silicon, or a concentrationof silicon in the fourth crystal region is less than the concentrationof silicon in the third crystal region.
 5. The device according to claim1, wherein at least a portion of a crystal of the third crystal regionis continuous with a crystal of the fourth crystal region.
 6. The deviceaccording to claim 1, wherein at least a portion of a crystal of thethird crystal region is continuous with a crystal of the first crystalregion.
 7. The device according to claim 1, further comprising: a fifthcrystal region provided between the first crystal region and the thirdcrystal region, the fifth crystal region including Al_(x5)Ga_(1-x5)N(0<x5≤1, x1<x5, and x3<x5).
 8. The device according to claim 7, whereinat least a portion of a crystal of the third crystal region iscontinuous with a crystal of the fifth crystal region.
 9. The deviceaccording to claim 8, wherein the at least a portion of the crystal ofthe fifth crystal region is continuous with a crystal of the firstcrystal region.
 10. The device according to claim 1, wherein a thicknessof the third crystal region is not less than 2 nm and not more than 20nm.
 11. The device according to claim 1, wherein a thickness of thethird crystal region is not less than 2 nm and not more than 10 nm. 12.The device according to claim 1, wherein the fourth crystal region doesnot include magnesium, or a fourth concentration of magnesium in thefourth crystal region is less than a first concentration of magnesium inthe first crystal region.
 13. The device according to claim 12, whereinthe fourth concentration is not more than ⅕ of the first concentration.14. The device according to claim 12, wherein a third concentration ofmagnesium in the third crystal region decreases along a firstorientation, the first orientation is from the first crystal regiontoward the fourth crystal region, and a third change rate of a logarithmof the third concentration with respect to a change of a position alongthe first orientation is greater than a fourth change rate of alogarithm of the fourth concentration with respect to a change of aposition along the first orientation.
 15. The device according to claim1, wherein a thickness of the fourth crystal region is not less than 10nm and not more than 200 nm.
 16. The device according to claim 1,further comprising: a substrate; and a sixth crystal region including anitride semiconductor including Al, the sixth crystal region beingbetween the substrate and the first crystal region.
 17. The deviceaccording to claim 1, further comprising: a first electrode; a secondelectrode; and a third electrode, a direction from a portion of thefirst crystal region toward the first electrode being along a firstdirection from the first crystal region toward the second crystalregion, a direction from an other portion of the first crystal regiontoward the second electrode being along the first direction, a seconddirection from the first electrode toward the second electrode crossingthe first direction, a position in the second direction of the thirdelectrode being between a position in the second direction of the firstelectrode and a position in the second direction of the secondelectrode.